High-resolution Reciprocal Space Mapping Research Articles

3-Dimensional Characterization of Polycrystalline Bulk Materials Using High-Energy Synchrotron Radiation

The implementation of 3-Dimensional X-Ray Diffraction (3DXRD) Microscopy at the Advanced Photon Source is described. The technique enables the non-destructive structural characterization of polycrystalline bulk materials and is therefore suitable for in situ studies during thermo-mechanical processing. High energy synchrotron radiation and area detectors are employed. First, a forward modeling approach for the reconstruction of grain boundaries from high resolution diffraction images is described. Second, a high resolution reciprocal space mapping technique of individual grains is presented.

Growth of dilute nitride alloys of GaInSb lattice-matched to GaSb

The growth of dilute nitride alloys of GaInSb by plasma-assisted molecular beam epitaxy is reported. The principle of lattice matching GaInNSb alloys to GaSb(0 0 1) substrates is demonstrated. High resolution X-ray diffraction rocking curves and reciprocal space maps indicate the high crystalline quality of the GaInNSb layers and illustrate a lattice match to GaSb with nitrogen and indium incorporations of 1.8% and 8.4%, respectively. The rms roughness of a nominally lattice matched GaInNSb/GaSb(0 0 1) layer was determined from atomic force microscopy to be ∼ 1.8 nm .

High-resolution XRD study of stress-modulated YBCO films with various thicknesses

High-quality epitaxial YBa 2Cu 3O 7− δ (YBCO) superconducting films with thicknesses between 0.2 and 2 μm were fabricated on (0 0 l) LaAlO 3 with direct-current sputtering method. The influence of film thickness on the structure and texture was investigated by X-ray diffraction conventional θ–2 θ scan and high-resolution reciprocal space mapping (HR-RSM). The films grew with strictly c-axis epitaxial, and no a-axis-oriented growth was observed up to a thickness of 2 μm. Lattice parameters of the YBCO films with different thicknesses were extracted from symmetry and asymmetry HR-RSMs. The X-ray lattice parameter method was used to determine the residual stress in YBCO films by measuring the a-, b-, c-axis strains, respectively. The results showed that YBCO films within thinner than 1 μm were under compressive stress, which was relieved increasing of film thickness. However, beyond 1 μm in thickness, YBCO films exhibited a tensile stress. Based on the experimental analysis, the variety of residual stresses in the films is mainly attributed to oxygen vacancies with thickness of YBCO film increasing.

Self-patterned oxide nanostructures grown by post-deposition thermal annealing on steppedsurfaces

We report here on the self-patterned growth of nanostructured epitaxial oxide thin filmsusing a bottom-up approach. Different stepped (0001) sapphire surfaces have been used astemplates to grow different nanostructured thin films of yttria-stabilized zirconiaelaborated by a chemical solution deposition technique, namely sol–gel dip-coating. Forsuch films, a morphological instability occurs during post-deposition thermal annealing thatforces the film to split into isolated islands. Taking advantage of this effect, three distincttypes of nanostructures have been produced depending on the step morphology of thesubstrate surface, and depending on the relative magnitude of the mean island diameter(D) and the meanterrace length (L). Flat-top islands, dome-shaped islands, tilted flat-top islands or self-organized arrays ofnano-islands can be generated. Atomic force microscopy was used to observenanostructures and quantitative characterizations of crystallographic and nanostructuralaspects were carried out by high-resolution x-ray diffraction reciprocal space mapping.

Asymmetric relaxation of SiGe∕Si(110) investigated by high-resolution x-ray diffraction reciprocal space mapping

Strain relaxation of SiGe∕Si(110) has been studied by x-ray reciprocal space mapping. To get information about the in-plane lattice mismatch in different directions, two-dimensional maps around, e.g., (260) and (062) reciprocal lattice points have been obtained from Si0.8Ge0.2∕Si(110) samples, which were exposed to different annealing conditions. The in-plane lattice mismatch was found to be asymmetric with the major strain relaxation observed in the lateral [001] direction. This was associated with the formation and propagation of dislocations oriented along [1¯10]. The relaxation of as-grown structures during postannealing is thus different from relaxation during growth, which is mainly along [1¯10].

Structural Characterization of BiFeO3 Thin Films by Reciprocal Space Mapping

High-resolution X-ray diffraction reciprocal space mapping (HRXRD-RSM) was applied for the structural characterization of epitaxial BiFeO3 thin film grown on SrRuO3-coated (001) SrTiO3 single crystal substrate with a variety of film thicknesses ranging from 15 to 500 nm. Distinguishing monoclinic structure from rhombohedral or tetragonal structures was accomplished with a set of HRXRD-RSMs measured for several hkl diffraction conditions. The BiFeO3 thin films showed both monoclinic and tetragonal structures depending on film thickness. Tetragonal structure was observed for film thicknesses below 50 nm and was highly strained due to epitaxial strain, while films with film thickness thicker than 50 nm showed monoclinic structure. No BiFeO3 thin films showed bulk rhombohedral structure. This structural change in BiFeO3 thin film may play an important role in the enhanced ferroelectricity observed.

Superconducting molybdenum nitride epitaxial thin films deposited on MgO and α-Al 2O 3 substrates by molecular beam epitaxy

Molybdenum nitride Mo 2N x films were grown on MgO(0 0 1) and on α-Al 2O 3(0 0 1) substrates by molecular beam epitaxy under nitrogen radical irradiation. X-ray photoelectron spectroscopy revealed that the composition of the film varied in the range of Mo 2N 1.4–Mo 2N 2.8 depending on the growth temperature. The deposition at 973 K gave well-crystallized films on both substrates. The high-resolution reciprocal space mapping by X-ray diffraction showed that the nitrogen-rich γ-Mo 2N crystalline phase (the composition: Mo 2N 1.4) was epitaxially grown on MgO at 923 K with a slight tetragonal distortion ( a = 0.421 and c = 0.418 nm) to fit the MgO lattice ( a = 0.421 nm). On α-Al 2O 3(0 0 1), nitrogen-rich γ-Mo 2N (Mo 2N 1.8) was grown at 973 K with (1 1 1) planes parallel to the substrate surface. X-ray diffraction analysis with a multi-axes diffractometer revealed that the γ-Mo 2N on α-Al 2O 3(0 0 1) had a slight rhombohedral distortion ( a = 0.4173(2) and α = 90.46(3)°). Superconductivity was observed below 2.8–3 K for the films grown at 973 K on MgO and on α-Al 2O 3(0 0 1).

On the interpretation of structural and light emitting properties of InGaN/GaN epitaxial layers grown above and below the critical layer thickness

In this contribution, the optical and structural properties of InGaN/GaN layers grown by metal-organic chemical vapour deposition (MOCVD) are studied. The main focus of this investigation is on the difference between microstructural and luminescence characteristics, for layers grown below and above the critical layer thickness (CLT) for elastic strain relaxation. By comparing the photoluminescence properties of samples grown under the same nominal conditions, except the deposition time, it is shown that in InGaN films grown above the CLT ( x), an additional lower energy secondary luminescence component emerges. Specifically, for an InGaN layer with x∼0.1, the energy splitting between the two components is about 160 meV. The surface of samples with thicknesses larger than CLT( x), are found to be rough with pronounced islanding occurring, indicating that a Stranski–Krastanow 2D to 3D growth mode transition takes place after the CLT. A detailed structural characterization by high-resolution reciprocal space mapping reveals that the appearance of 3D islands is associated with elastic strain relaxation. Strong lateral and depth variations of the strain field associated to a peculiar 2D/3D growth mode can explain structural and optical properties, which are typically considered “anomalous” and frequently ascribed to phase segregation effects in InGaN. A simple calculation based on elastic strain relaxation, accounts for the observed energy splitting on the photoluminescence.

Characterization of Coherent Si1−xGex Island Superlattices on (100) Si

ABSTRACTWe examine the nanostructural properties of Si/Si1−xGex island superlattices with 0.37 < x < 0.56 grown at 620-640 °C by molecular beam epitaxy. Analytical transmission electron microscopy (TEM) shows that during growth Ge atoms migrate towards the center of the large islands to maintain epitaxial growth and that the most regular structures are obtained at higher Ge composition when the built-in strain is also higher. High-resolution x-ray reciprocal space mapping shows that these heterostructures remain pseudomorphic and that the undulations are aligned along [010] directions and exhibit a long-range coherence and vertical correlation as revealed by the presence of strong satellites in a wave vector direction parallel to the surface. Raman spectra of these samples exhibit the usual Ge-Ge, Si-Ge and Si-Si vibrational modes. When compared with planar Si/Si1−xGex superlattices the Ge-Ge and Si-Ge Raman peaks are shifted to lower frequencies indicating an average alloy composition that is approximately x = 0.1 less than the nominal values, which is in general agreement with analytical TEM.

Band gap reduction in GaNSb alloys due to the anion mismatch

The structural and optoelectronic properties in GaNxSb1−x alloys (0⩽x<0.02) grown by molecular-beam epitaxy on both GaSb substrates and AlSb buffer layers on GaAs substrates are investigated. High-resolution x-ray diffraction (XRD) and reciprocal space mapping indicate that the GaNxSb1−x epilayers are of high crystalline quality and the alloy composition is found to be independent of substrate, for identical growth conditions. The band gap of the GaNSb alloys is found to decrease with increasing nitrogen content from absorption spectroscopy. Strain-induced band-gap shifts, Moss-Burstein effects, and band renormalization were ruled out by XRD and Hall measurements. The band-gap reduction is solely due to the substitution of dilute amounts of highly electronegative nitrogen for antimony, and is greater than observed in GaNAs with the same N content.

Goniometric and topographic characterization of synthetic IIa diamonds

Synthetic type IIa diamonds were characterized using rocking curve measurements and x-ray topography with quasi-plane-wave incidence, and high resolution reciprocal space mapping. The rocking curves of the (111) crystals were comparable to the theoretical ones. Some crystals had wider rocking curves because of the higher density of stacking faults. Even the highest quality crystal did not give the theoretical rocking curve due to dislocations and strain bands. On the other hand, the (100) crystals had nearly perfect area. The difference was attributed to the orientation of the seed crystal.

Microstructural and morphological properties of homoepitaxial (001)ZnTe layers investigated by x-ray diffuse scattering

The microstructural and morphological properties of homoepitaxial (001)ZnTe layers grown by metalorganic vapor phase epitaxy at a temperature (TG) between 325°C and 400°C are investigated by x-ray diffuse scattering. High resolution reciprocal space maps (RSMs) recorded close to the ZnTe (004) Bragg peak show different diffuse scattering features that can be ascribed to (i) the specific surface morphology of the sample, and (ii) the presence of extended lattice defects in the epilayers. One kind of cross-shaped diffuse scattering streaks, appearing for TG⩾350°C, extend along ⟨111⟩ directions and can be attributed to stacking faults (SFs) occurring at the epilayer–substrate interface, within the epilayers. The SF diameter was estimated around 200–300nm, while their density increases with TG. Another kind of cross-shaped diffuse streaks, inclined at an angle β≈80° with respect to the ⟨110⟩ in-plane direction, arises from the morphology of epilayers grown above 360°C, their surfaces being covered by pairs of pyramidal hillocks up to a density of 106–107cm−2. Atomic force microscopy (AFM) measurements showed that the apex angles of the pyramids compare well with the value of 2β. The hillock formation is ascribed to Te adatoms experiencing a Schwoebel potential barrier at the step edges around pairs of partial dislocations (dipoles) bounding the SFs. In a quite narrow growth temperature interval around 350°C no β-crossed diffuse streaks are instead observed in the RSMs, indicating a smooth ZnTe surface. Finally, at a lower growth temperature (TG=325°C) a diffuse scattering intensity distribution defined by an angle γ≈63° with respect to the ⟨110⟩ in-plane direction is observed, corresponding to a dense ridging of the epilayer surface along the perpendicular direction. Both RSM analysis and AFM measurements indicate that the ridge sidewalls are {113} planes.

Phonon mode behavior in strained wurtziteAlN∕GaNsuperlattices

We have studied phonons in $\mathrm{Al}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ superlattices with different periods but a constant well-to-barrier ratio using a combination of infrared spectroscopic ellipsometry and Raman scattering spectroscopy. The strain evolution in the superlattice structures is assessed by high-resolution x-ray diffraction and reciprocal space mapping. We have identified ${E}_{1}(\mathrm{TO})$, ${A}_{1}(\mathrm{LO})$ and ${E}_{2}$ localized, and ${E}_{1}(\mathrm{LO})$ and ${A}_{1}(\mathrm{TO})$ delocalized superlattice modes. The dependencies of their frequencies on in-plane strain are analyzed and discussed, and the strain-free frequencies of the superlattice modes are estimated. A good agreement between theory and experiment is found in the case of GaN localized modes, while large deviations between theoretically estimated and experimentally determined frequency shifts are observed for the AlN localized modes. The delocalization effect on the ${A}_{1}(\mathrm{TO})$ and ${E}_{1}(\mathrm{LO})$ phonons, as well as the free-carrier effect on the ${E}_{1}(\mathrm{LO})$ phonon are also discussed.

Highly localized strain fields due to planar defects in epitaxial SrBi2Nb2O9 thin films

Thin films of (00l) oriented SrBi2Nb2O9 epitaxially grown on SrTiO3 by sol-gel spin coating have been studied by means of high-resolution x-ray diffraction reciprocal space mapping. It is shown that these materials contain highly localized heterogeneous strain fields due to imperfect stacking faults (i.e., faults that do not propagate throughout the crystallites building up the film). In the film plane, the strain fields are confined to 11 nm wide regions and characterized by a vertical displacement of 0.18c (where c is the cell parameter) showing that the stacking faults are mainly composed of one additional (or missing) perovskite layer. Prolonged thermal annealing at 700 °C strongly reduces the density of stacking faults and yields a more uniform strain distribution within the film volume without inducing significant grain growth.

Spontaneous polarization change with Zr∕(Zr+Ti) ratios in perfectly polar-axis-orientated epitaxial tetragonal Pb(Zr,Ti)O3 films

Epitaxial Pb(Zr,Ti)O3 (PZT) films 50 and 250nm thick with the Zr∕(Zr+Ti) ratio from 0.13 to 0.65 were grown on (100)cSrRuO3∕∕(100)SrTiO3 substrates at 540°C by pulsed-metalorganic chemical vapor deposition. The crystal orientation, lattice parameter, axial angle, and relative volume fraction of constituent domains were determined by high-resolution x-ray diffraction reciprocal space mapping. We grew (100) and∕or (001)-oriented epitaxial PZT films for the whole Zr∕(Zr+Ti) ratio. A tetragonal single phase was obtained for the Zr∕(Zr+Ti) ratio ranging from 0.13 to 0.54 and 0.19 to 0.45 with the 50 and 250nm thick films, respectively, while mixed phases of a tetragonal and rhombohedral from 0.45 to 0.60 for the 250nm thick films was obtained. For the films consisting of a tetragonal single phase, lattice parameters of a- and c-axes and their ratio (c∕a) were almost the same as the reported data for the PZT powder, suggesting a small amount of residual strain in them. In addition, perfectly polar-axis (c-axis)-oriented tetragonal films were obtained for the 50nm thick films, while an ∼70% c-axis-orientation for the 250nm thick ones was obtained. Polarization–electric-field hysteresis loops with a good square shape were observed for all films. Also, spontaneous polarization (Ps) values that were directly measured for the 50nm thick films and estimated for the 250nm thick films taking into account the volume fraction of the polar-axis-orientated domain were closely in line with the Zr∕(Zr+Ti) ratio, and they decreased as the ratio increased. The square of the estimated Ps was found to be proportional to the crystal distortion, c∕a−1. These results are essential not only for understanding the origin of the large Ps of PZT but for the property design of the ferroelectric devices using PZT.

Selective area growth of InAs quantum dots formed on a patterned GaAs substrate

We describe the growth and characterization of InAs quantum dots (QDs) on a patterned GaAs substrate using metalorganic chemical vapor deposition. The QDs nucleate on the (001) plane atop GaAs truncated pyramids formed by a thin patterned SiO2 mask. The base diameter of the resulting QDs varies from 30 to 40nm depending on the size of the mask. With specific growth conditions, we are able to form highly crystalline surface QDs that emit at 1.6μm under room-temperature photopumped conditions. The crystalline uniformity and residual strain is quantified in high-resolution transmission electron microscopy images and high-resolution x-ray reciprocal space mapping. These strained QDs may serve as a template for selective nucleation of a stacked QD active region.

Mathematical analysis of the high-resolution reciprocal space map

A reciprocal space map recorded by means of a high-resolution diffractometer is a function of two angular variables: ω and 2θ defined as angular position of the studied crystal and position of an analyser, respectively. For comparative investigations of different samples the inter-planar spacing distribution function and the lattice reflecting plane orientation distribution function are more useful. In this paper we present the integral equations, which allow one to obtain these functions through the integration of the reciprocal space map matrix. An example of the mathematical analysis of the reciprocal space map for GaAs single crystal recorded by means of a Philips MRD high-resolution diffractometer is described and well-defined analytical formulae for the apparatus functions are given.

Strain evolution and phonons in AlN/GaN superlattices

ABSTRACTAlN/GaN superlattices (SLs) with different periods grown on GaN buffer layers were studied by infrared spectroscopic ellipsometry (IRSE), Raman scattering (RS) and highresolution reciprocal space mapping (RSM). The lattice parameters and the degree of strain in the GaN buffer and the SL constituents were determined. Phonon modes originating from the buffer layer and the SL sublayers were identified and their frequency shifts were correlated with the strain state of the films.

Structural characterization and 90° domain contribution to ferroelectricity of epitaxial Pb(Zr0.35,Ti0.65)O3 thin films

Remanent polarizations (Pr) of 200-nm-thick epitaxial Pb(Zr0.35,Ti0.65)O3 (PZT) thin films deposited on (001), (110), and (111) SrTiO3 (STO) substrates coated with SrRuO3 (SRO) were compared to the domain configurations that were precisely and quantitatively characterized by high-resolution x-ray diffraction reciprocal space mapping (HRXRD-RSM). (001)/(100), (101)/(110), and (111) oriented domains were obtained for films grown on (001), (110), and (111) STO substrates coated with SRO, respectively. HRXRD-RSM showed that the films grown on (001) and (110) STO substrates mainly consisted of (001) and (101) domains, although they also included about 32% and 25% of (100) and (110) domain, respectively. Tilt growths in the domains were found except for the (001) domain. The tilt growths in the (100), (101), and (110) domains were attributed to the geometrically induced tilt by the 90° domain that had {101} domain walls. On the other hand, the tilt in the (111) domain was attributed to the misfit strain relaxation by introducing tilt growth in the domain but not due to the 90° domain. The Pr ratios of films having different domain configurations were well explained by the estimated Pr ratios from the volume fractions of the domains, based on the assumption that the 90° domain was not reoriented by the externally applied electrical field and did not contribute to the measured Pr values. This indicates that the 90° domain is strongly pinned in epitaxial 200-nm-thick PZT films and the 180° domain switching is the dominant contribution to the total remanent polarization.

Pulsed laser deposition of novel nitride solid solution films Ni xTi 1− xN y on MgO(0 0 1)

A novel nitride solid solution, Ni x Ti 1− x N y (0≤ x≤0.31, 0.59≤ y≤1) was grown on MgO(0 0 1) using a pulsed laser deposition (PLD) method combined with RF nitrogen radical irradiation. Compressed disks made of mixtures of titanium hydride and nickel metal were used as the PLD targets. Simple Ti–Ni alloyed targets gave no crystalline nitride. High-resolution X-ray diffraction reciprocal space mapping revealed that the deposited films were epitaxially grown on MgO(0 0 1) and that the lattice of the films shrunk to fit the lattice of MgO(0 0 1) in the plane. The lattice constant perpendicular to the plane decreased linearly as the Ni content increased up to x=0.31, demonstrating the incorporation of Ni atoms into the crystal structure. These thin films were metallic conductors. X-ray photoelectron spectra showed that the substitution of Ti for Ni brought about the formation of large amounts of nitrogen vacancies.